In fuel burners such as furnaces where the main burner is lit by a pilot burner, it is necessary for obvious reasons to assure that the pilot burner is lit before the main burner fuel valve is opened. This is true whether a standing pilot or intermittent pilot is involved. While there are many different types of sensing operations which can reliably detect presence of a pilot flame, one which is preferred senses the flicker frequency of typical pilot flames. This flicker is a periodic variation of the intensity or amplitude of the infrared, visible, or ultraviolet radiation produced by the burning of the fuel sustaining the pilot flame. The flicker frequency of this radiation in most cases has a component in the range of 13-17 hz. This characteristic is fairly independent of the fuel and the size of the pilot flame.
In the past an analog circuit has been used to sense for presence of this flicker in the intensity of the radiation emanating from the location of the pilot flame. However, the relatively large size of components for an analog-based flame sensing system for the low frequencies involved here, cannot be easily included on a single circuit board with the smaller digital and logic-based circuits which are now more and more often being used to implement other functions of typical fuel burners. This necessitates a separate flame sensor board or a larger single board with a larger power supply, resulting in turn in undesirable expense and inconvenience.